The present invention relates to block copolymer melt-processable compositions such as adhesives, pressure sensitive adhesives, sealants, elastomers, other hot melt processable compositions, to methods of their preparation, and to articles having a coating of such a composition applied thereto.
Among adhesive chemistries, poly(meth)acrylates (e.g., polymers derived at least in part from one or more methacrylate monomer or acrylate monomer) are one of the most prominent. (Meth)acrylates have evolved as a preferred class of adhesives due to their durability, permanence of properties over time, and versatility of adhesion, to name just a few of their benefits.
Traditionally, adhesives such as (meth)acrylates have been provided in organic solvent for processing, application, or other incorporation into a larger product. Solvent based adhesives can be applied to a substrate and the solvent can be removed, leaving behind the adhesive.
Hot-melt adhesives advantageously reduce or eliminate the use of organic solvents in adhesives and their processing. Hot-melt adhesive systems are essentially 100% solid systems. Usually, such systems contain no more than about 5% organic solvent or water, more typically no more than about 3% organic solvent or water. Most preferably, such systems are free of organic solvent and water. Advantageously, by reducing the use of organic solvents, special handling concerns associated with the use of organic solvents are also reduced.
Melt-processable block copolymer materials have been prepared, as described in PCT International Publication Number WO 00/39233. These block copolymers are described as generally contemplated to include homopolymer or copolymer blocks. According to this publication and other prior art techniques, to provide a block copolymer with melt-processing capability, the molecular weight of homopolymer end blocks may be relatively low. The relatively low molecular weight of the end blocks can still allow for useful and acceptable cohesive strength, but the ability to use relatively higher molecular weight end blocks, without losing melt-processing capabilities, could be advantageous by further improving other properties of a block copolymer composition such as cohesive strength.
Block copolymers contain at least two different polymeric xe2x80x9cblocksxe2x80x9d that cause the bulk block copolymer to exhibit desired properties. (The term xe2x80x9cblock copolymerxe2x80x9d is used herein to describe a block copolymer on a molecular scale, and also for convenience to reference a block copolymer-containing composition or xe2x80x9cbulkxe2x80x9d block copolymer). Typically, one block, the end block or xe2x80x9cAxe2x80x9d block, is a relatively high glass transition temperature polymeric block that provides structural and cohesive strength within use temperature ranges. The xe2x80x9cBxe2x80x9d block or blocks, which may typically constitute the middle or core of the block copolymer, have a relatively lower glass transition temperature and provide elastomeric properties. The chemistry of the B block can also affect properties of the block copolymer composition including glass transition temperature and modulus, which relate to tackiness of the composition.
The polymeric blocks interact with each other in a bulk composition differently at different temperatures, providing useful temperature-controlled properties. At low temperatures, e.g., use temperatures, e.g., temperatures below the glass transition temperature of the end A blocks and above the glass transition temperature of the B blocks (e.g., for pressure sensitive adhesive and elastomer compositions, typically below 100xc2x0 C. and above xe2x88x9250xc2x0 C.), the different blocks organize into ordered A and B phases, or xe2x80x9cphase separate,xe2x80x9d within the bulk block copolymer composition. For compositions containing less than about 50 weight % of the A block, typically microdomains of discontinuous A block are formed within a continuous phase of B block. The A domains provide rigidity and strength within the lower modulus continuous B phase, for a desirable combination of properties. At higher temperatures, e.g., at a temperature greater than the Tg of an A block, e.g., greater than 100xc2x0 C. to about 200xc2x0 C., the bulk block copolymer can be melt processed. In a favorably designed block copolymer, the thermal energy imparted to the bulk block copolymer at these temperatures is sufficient to disrupt the ordered multiphase morphology and create disorder within the block copolymer composition. The disordered composition does not retain the strength of the ordered microdomains and as a result can flow and be xe2x80x9cmelt processedxe2x80x9d relatively easilyxe2x80x94melt-processable block copolymer compositions have a viscosity upon melting that allows the compositions to be melt-processed (e.g., applied to a substrate). Upon cooling, the composition returns to the ordered, strengthened, multi-phase morphology.
FIG. 2 illustrates thermal behavior of a block copolymer of Example 1 over a range of temperatures such that the different regions of block copolymer viscoelastic behavior could be accessed. Gxe2x80x2 (storage modulus), Gxe2x80x3 (loss modulus), and tan ∫ (the ratio Gxe2x80x3/Gxe2x80x2) are plotted in the figure as a function of temperature. These dynamic mechanical measurements were conducted using a rheometer in a shear geometry. At very low temperatures ( less than xe2x88x9250xc2x0 C.), the entire block copolymer is in a glassy state and the material is predominantly elastic (Gxe2x80x2 greater than  greater than Gxe2x80x3). A precipitous drop is observed in Gxe2x80x2 over a temperature range (ca. xe2x88x9250xc2x0 C. to ca. xe2x88x9210xc2x0 C.) and a peak in tan ∂ is observed which is associated with the Tg of the B block. A plateau in Gxe2x80x2 is observed from ca. 0xc2x0 C. to ca. 100xc2x0 C. and is attributed to the entanglements of the B block polymer chains. Above ca. 100xc2x0 C., Gxe2x80x2 starts dropping sharply due to the onset of flow in the system and as the Tg of the A block is exceeded. Accordingly, the viscoelastic response is dominated by Gxe2x80x3 in this flow region (Gxe2x80x3 greater than Gxe2x80x2) and a steep increase in tan ∂ (=Gxe2x80x2/Gxe2x80x2) is observed. It is in this xe2x80x9cflow regionxe2x80x9d of the viscoelastic curve that melt processing is often conducted.
The temperature at which meltflow occurs is referred to herein as the meltflow temperature. One convenient measurement of meltflow temperature that can be used for purposes of this description is that the meltflow temperature of a block copolymer is the temperature at the intersection of Gxe2x80x2 and Gxe2x80x3 in the flow region of the viscoelastic curve.
FIG. 1 shows a plot of Gxe2x80x2 versus temperature for a variety of block copolymers. The meltflow temperature progressively increases in this case for copolymers identified herein as Examples 1, 2, 3, and 4. The flow region could not be accessed for Example 5, even when heated to 240xc2x0 C., and so it would be difficult to hot melt process this material without causing thermal degradation or without the use of other processing aids.
Different features of the molecular structures of the A and B polymers have been found to affect properties of bulk block copolymers such as tackiness (or non-tackiness), meltflow temperature, modulus, Tg, and cohesive strength. These features include the molecular weight of an A block polymer or a B block polymer and the ratio of the molecular weight of the A block polymer to the molecular weight of the B block polymer (MWA:MWB). In general, higher molecular weight A blocks increase cohesive strength of a bulk block copolymer, but will also increase meltflow temperature (for a given MWB), which may not be desired. The ratio of MWA to MWB can have a significant effect on which phase is the continuous phase, the A block or the B block. This in turn can alter the properties of the block composition. Preferred block compositions have a continuous B block, and it can therefore be preferred to keep the ratio of MWA to MWB in a range to maintain the continuous B block.
Often, it is desirable to control (e.g., increase or decrease) the meltflow temperature of a block copolymer, while preferably retaining other desirable properties of the block copolymer. For instance, meltflow temperature of a block copolymer may be desirably reduced if degradation of a polymer is an issue (due to excessive processing temperature) or where a composition is coated onto a temperature sensitive substrate. At other times, meltflow temperature may desirably be increased, for example during co-extrusion of a block copolymer with another material having a higher meltflow temperature, to better match processing characteristics like viscosity.
Past methods of adjusting meltflow temperature have involved adjusting molecular weight of the A and/or B blocks. As described above, increasing or decreasing molecular weight of the blocks can affect (increase or decrease, respectively) the meltflow temperature of the bulk block copolymer. Unfortunately, the increase or decrease in molecular weight of the blocks will have a direct corresponding effect on other properties of the bulk block copolymer such as cohesive strength or elastomeric characteristics, which can be undesirable. Also unfortunately, the change in block molecular weight can cause an unintended and undesirable change in the ratio of MWA:MWB, which can further negatively affect one or more properties of the bulk block copolymer.
According to the invention, meltflow temperature of a block copolymer can be selectively controlled and adjusted by choosing the A block to be a copolymer, and by selecting the composition of the A block copolymer, i.e., the monomeric units that make up the A block copolymer, to achieve control of the meltflow temperature while preferably maintaining, at least to a desired extent, preferably a substantial extent, other desired properties of the block copolymer.
The invention specifically allows selection and adjustment of meltflow temperature of a block copolymer by adjusting the level of compatibility (or miscibility) between polymeric A and B blocks of a block copolymer, by selecting the composition of copolymeric A blocks. The composition of the A blocks is selected to include a first monomeric unit that provides strength and a desired glass transition temperature, and a second monomeric unit that desirably alters the meltflow temperature of the block copolymer without having to also significantly affect MWA.
The invention can achieve advantageous rheological properties such as the ability to adjust, i.e., selectively increase or reduce, a meltflow temperature, while preferably still providing other desirable properties of a block copolymer such as high cohesive strength. By selecting the composition of an A block copolymer instead of or in addition to molecular weight of the A block, to adjust meltflow temperature, the level of compatibility between A blocks and B blocks can be selectively adjusted and controlled (increased or decreased) without requiring a change in MWA or the ratio of MWA to MWB. Preferably, this can allow adjustment and control of meltflow temperature without causing the same degree of negative effects otherwise created by changing, e.g., increasing, MWA. Optionally, in preferred embodiments, the use of copolymeric end block to alter meltflow temperature can allow the use of relatively higher molecular weight end block compared to the use of homopolymeric end block, because meltflow temperature can be maintained based on the composition of the copolymeric A block, even at higher molecular weights. A higher molecular weight A block may allow for improved cohesive strength, while still retaining a desired (e.g., low) meltflow temperature. Additionally, the molecular weight ratio of the A block copolymer to the B block polymer does not have to he affected and the B block can be maintained as the continuous phase.
The invention specifically contemplates block copolymers having at least one relatively high glass transition temperature copolymeric end block (xe2x80x9cA blockxe2x80x9d) and at least one relatively lower glass transition temperature polymeric B block, e.g., at the interior of the block copolymer. The end blocks can be collectively referred to as xe2x80x9cAxe2x80x9d blocks, but all A blocks of a block copolymer molecule or composition do not necessarily have chemically identical or similar composition, and while compositions of the invention include block copolymer with copolymeric A blocks, not all A blocks within a block copolymer or block copolymer composition are required to be copolymeric. Some may be homopolymers. B blocks may have the same or different composition and molecular weight, and may be homopolymeric or copolymeric.
Preferably, the block copolymer composition can comprise at least one of an (A-B) diblock copolymer, (A-B-A) triblock copolymer, an xe2x80x94(A-B)nxe2x80x94 multiblock copolymer, an (A-B)nxe2x80x94 star block copolymer, and may be a combination of two or more of these. Particularly preferred are linear (A-B-A) triblock and (A-B)nxe2x80x94 star block structures. In certain embodiments of the invention, the block copolymer can be a (meth)acrylate block copolymer, meaning that at least one of the A and B blocks is derived from one or more (meth)acrylate monomer.
A blocks of a particular copolymer molecule or of a bulk copolymer composition can be copolymers independently derived from a monoethylenically unsaturated monomer that as a homopolymer would have a glass transition temperature (Tg) of greater than about 20xc2x0 C., preferably about 20xc2x0 C. to about 200xc2x0 C., and more preferably about 50xc2x0 C. to about 150xc2x0 C. The copolymer can be prepared from a first monoethylenically unsaturated monomer and a second monoethylenically unsaturated monomer, to comprise respective first and second monomeric units. A first monomeric unit can be selected to provide the described Tg of the A block. Certain preferred first monomeric units can be derived from linear and branched (meth)acrylate monomers such as methyl methacrylate, from ethylenically unsaturated cycloaliphatic monomers (e.g., cyclohexyl methacrylate, isobornyl methacrylate, or others) or styrenes, or from ethylenically unsaturated aromatic monomers (e.g., aromatic (meth)acrylates). A second monomeric unit can be selected to adjust melt processing properties of the block copolymer, preferably without substantially negatively affecting other desired properties of the bulk block copolymer. Certain preferred second monomeric units can be derived from polymerizable, substituted or unsubstituted, ethylenically unsaturated aromatic or cycloalkyl monomers, e.g., vinyl-functional or (meth)acrylate functional cycloalkyl or aromatic monomers such as styrene, cyclohexylmethacrylate, isobomylmethacrylate, and the like. Any useful relative amounts of the first and second monomers can be used, and additional monomers (e.g., a third or fourth monomer) can also be included in a copolymeric A block if desired, although, it can be preferred for simplicity that only a small number of monomers make up the A block copolymer, e.g., two or three.
According to certain methods of the invention, the copolymeric composition (optionally in combination with molecular weight) of an A block copolymer can be selected to control a meltflow temperature of the block copolymer, preferably while at least maintaining or perhaps even improving other desired block copolymer, structural features and their dependent properties, such as MWA:MWB of the block copolymer molecule, or modulus or cohesive strength of a bulk block copolymer. A first monomeric unit can be identified which as a homopolymeric A block having molecular weight MWA, in a particular block copolymer in combination with specified B blocks, would produce a block copolymer with certain properties of, e.g., meltflow temperature, modulus, or cohesive strength. According to the invention, the homopolymeric A block can be replaced with a copolymeric A block of the same molecular weight, that contains that same first monomeric unit in combination with a second monomeric unit. The copolymeric A block can contain second monomeric units to desirably affect compatibility between the A and B blocks, thereby increasing or decreasing the meltflow temperature of the bulk block copolymer, while preferably not substantially negatively affecting at least one other property of the block copolymer such as MWA:MWB (and related properties), cohesive strength, or another important property of the block copolymer. According to this embodiment of the invention, the type and amount of second monomer can be selected to desirably increase or decrease meltflow temperature of the block copolymer compared to a block copolymer that is otherwise similar (e.g., having other of the same molecular properties such as MWA:MWB and molecular weight of the A and B blocks) but does not contain the second monomeric units (i.e., the A block is a homopolymer of the first monomer). Also, use of copolymeric blocks in a block copolymer, according to the invention, can allow improvement of a block copolymer having good meltflow properties but poor adhesive properties. For example, including a copolymeric A block having a higher MWA, in such a copolymer, can improve adhesive properties while retaining meltflow properties.
Preferred A block copolymers can each have a weight average molecular weight of less than about 100,000 grams per mole e.g., from about 3,000 to about 50,000 grams per mole.
Typically, the B block can be a polymer derived from a monoethylenically unsaturated monomer, that as a homopolymer has a glass transition temperature (Tg) of less than about 20xc2x0 C., preferably about xe2x88x9270xc2x0 C. to about 20xc2x0 C., and more preferably xe2x88x9260xc2x0 C. to about 0xc2x0 C. Preferably, the monoethylenically unsaturated monomer can be a (meth)acrylate monomer. B blocks can have a weight average molecular weight of about 30,000 to about 500,000 grams per mole, more preferably about 50,000 to about 200,000 grams per mole.
The block copolymer can be useful alone or in combination with other polymeric or non-polymeric materials, preferably in any of a variety of melt-processable, e.g., xe2x80x9cthermoplastic,xe2x80x9d polymeric compositions, such as adhesives, sealants, elastomers, reinforced rubber, and other polymeric compositions. In one embodiment, the block copolymer can be used as or included in a melt-processable adhesive composition, e.g., a pressure sensitive adhesive. Examples of such adhesive compositions may contain the block copolymer as the only or essentially the only elastomeric component, e.g., may consist of or consist essentially of the block copolymer and optional adhesive composition additives such as a tackifier: e.g., 100 parts by weight of at least one block copolymer comprising at least two copolymeric A blocks and at least one homopolymeric or copolymeric B block, and 10 to 200 parts by weight of at least one tackifier based on total weight of the block copolymer.
Exemplary adhesive compositions of the invention can be pressure sensitive adhesive (PSA) compositions. However, the invention also contemplates other adhesive compositions such as heat-activatable adhesive compositions, as well as non-adhesive compositions.
Preferred adhesive compositions can be formulated to have a cohesive strength of at least about 2,000 minutes when measured according to ASTM D 3654, more preferably a cohesive strength of at least about 5,000 or 6,000 minutes when measured according to ASTM D 3654, and even more preferably, at least about 10,000 minutes when measured according to ASTM D 3654.
Broad formulation latitude is possible in block copolymer compositions of the invention while maintaining melt-processability and processing efficiency. For example, adhesives such as PSAs are obtainable even when elastomeric components in the composition consist of or consist essentially of the block copolymer. Thus, blending of more than one elastomeric component is not required to produce adhesive compositions according to the invention.
An aspect of the invention relates to a method of controlling meltflow temperature of a poly(meth)acrylate block copolymer, the meltflow temperature of the block copolymer being in the range from 50xc2x0 C. to 250xc2x0 C. The method comprises providing block copolymer comprising at least one low glass transition temperature polymeric block and at least one high glass transition temperature copolymeric end block comprising first monomeric units and second monomeric units, and selecting the amount and type of the second monomeric units to selectively increase or decrease meltflow temperature of the block copolymer compared to a block copolymer that is otherwise similar but does not contain the second monomeric units.
Another aspect of the invention relates to a melt-processable poly(meth)acrylate composition comprising block copolymer. The block copolymer comprises at least one high glass transition temperature copolymeric end blocks, and at least one low glass transition temperature polymeric block. The high glass transition copolymeric block comprises first monomeric units and second monomeric units, the second monomeric units increasing or decreasing meltflow temperature of the block copolymer compared to a similar block copolymer that does not contain the second monomeric units, and the meltflow temperature of the block copolymer is in the range from 50xc2x0 C. to 250xc2x0 C.
Still another aspect of the invention relates to a melt processable, thermoplastic poly(meth)acrylate block copolymer. The copolymer comprises at least one soft polymeric block, and at least two hard copolymeric end blocks having a glass transition temperature of from 20xc2x0 C. to 200xc2x0 C. and comprising first monomeric units selected from the group consisting of linear and branched alkyl(meth)acrylates, cycloaliphatic monomeric units, and aromatic monomeric units, and second monomeric units selected from the group consisting of cycloaliphatic monomeric units, aromatic monomeric units, and low glass transition temperature linear or branched alkyl acrylate or alkyl methacrylate monomeric units. The block copolymer has a meltflow temperature from 50xc2x0 C. to 250xc2x0 C.
Another aspect of the invention relates to a melt processable, thermoplastic block copolymer. The block copolymer comprises at least one soft polymeric block, and at least two hard copolymeric end blocks having a glass transition temperature of from 20xc2x0 C. to 200xc2x0 C. and comprising first monomeric units selected from the group consisting of ethylenically unsaturated polymerizable cycloaliphatic monomeric units, and second monomeric units selected from the group consisting of ethylenically unsaturated polymerizable aromatic monomeric units. The block copolymer has a meltflow temperature in the range from 50xc2x0 C. to 250xc2x0 C.